A series of standard alloys (known as (H-type alloys) have been developed for service in industrial furnaces and similar installations requiring moderately low hot strength as well as, almost always, resistance to attack by some combination of hot gases. These alloys almost always contain intentional additions of up to 0.9% by weight of carbon, because they derive much of their hot strength from the cabides that are thus formed. These alloys are generally referred to as heat-resistant alloys even though they must almost always resist the deleterious effects of some combination of hot gases.
While carbon content is sometimes increased in heat resistant alloys there are limits to the beneficial effects of intentional carbon additions. In any given grade of heat resistant alloy, increasing carbon levels eventually reduces hot ductility, thermal fatigue strength, and thermal and mechanical shock resistance as well as melting point. Accordingly, carbon levels in those alloys are generally less than about 1% by weight.
These standard heat resistant alloys may be generally contrasted in several ways to stainless steels and related corrosion resistant alloys intended primarily for service in various corrosive fluids and other substances at temperatures below a few hundred degrees Farenheit. For example, carbon is detrimental in most corrosive service so that corrosion resistant alloy specifications may allow maximum carbon levels of 0.08%, 0.05%, 0.03% or even less.
Thus, while some stainless steels may be otherwise quite similar to some heat resistant alloys with respect to elemental analysis, the very low carbon levels of the former cause them to have much lower hot strengths than the related heat resistant grades.
By far the most widely employed corrosion resistant alloys are the 18% Cr-8% Ni types. However, the analogous higher-carbon heat resistant grades are not much employed in furnace and similar applications, in large measure because they tend to corrode rapidly at temperatures above about 1650.degree. F. Since most furnace applications involve service well above this temperature, by far the most commonly employed grades of heat resistant alloys are the 25% Cr-12% Ni and 25% Ni types, which are known in their castings form as types HH and HK respectively. When greater resistance to thermal fatigue, thermal shock or carburization is required, the more expensive HT grade of 35% Ni-16% Cr is often employed. The HP grade, or 35% Ni-25% Cr is somewhat of a compromise between the HK and HT grades and combines good resistance to both oxidizing and carburizing atmospheres with higher hot strengths in the 1800.degree. to 2000.degree. F. range.
Another contrasting and somewhat specialized series of alloys generally referred to as superalloys owe their development to the advent of the gas turbine or jet engine. The most demanding service in these engines is found in the rotary turbine blades with equal corrosion resistance but somewhat less hot strength being required by the matching stator vanes. In these engines improved fuel efficiency came with higher operating temperatures. In the 1945-55 period of their development, in which blade temperatures increased from about 1400.degree. F. to about 1650.degree. F., chromium levels of about 15% to 22% provided sufficient resistance to hot gas corrosion at operating temperatures.
After 1955 or thereabout, a continuing trend toward lower chromium content was established. As a result, improvements in these blade alloys followed two paths initially, namely, modified cobalt-base alloys and nickel-base alloys. Cobalt-base alloys having increased hot strength were developed by formation of increasing amounts of carbides. But the hot strengthening effects of chromium carbides is somewhat limited so that cobalt-base alloys began to employ ever increasing quantities of the more effective carbide-forming elements molybdenum, tungsten, columbium, or even tantalum. The parallel development of improved nickel-base alloys was characterized by the development of hot strength principally by formation of so-called gamma-prime-phase precipitates, which are complex compounds of nickel with various quantities of titanium and aluminum. Eventually, the effects of the two methods of achieving hot strength were partially combined so that gamma prime-forming titanium and aluminum was present along with the carbide-forming elements with various proportions of nickel and cobalt.
Unfortunately, the carbide-forming elements as well as the gamma prime-forming elements all tend to reduce the matrix structural stability of these alloys. Therefore, as operating temperature demands increased, higher quantities of these two classes of elements were required to produce sufficient hot strengths at the higher application temperatures. In order to maintain matrix structural stability, chromium contents were reduced to levels of about 12%, 10%, 9%, 8% or even 6%. Since aluminum helps confer hot gas corrosion resistance, the increase in aluminum content to the 4% to 8% levels somewhat offset the deficiency resulting from decreased chromium content. On the negative side, however, such alloys must be melted and cast in vacuum or inert gas atmospheres due to their high aluminum contents as well as to the higher titanium content. A further problem is that vacuum or inert gas melting and casting processes are far too expensive for production of furnace and similar industrial parts. Hence, the superalloys developed for gas turbines or jet engines have turned out to the quite unsuited for most other industrial applications.
As a result, the need for alloys having improved Properties over those of the standard H-type alloys at reasonable cost, while widely recognized for decades remains unfilled. Yet, the materials performance demands of reforming, ethylene pyrolysis, coal gasification, iron ore reduction and other high temperature processes, are requiring and will continue to require heat resistance Properties beyond those of the HK-type alloys, which has held the major share of the market in the past. Specifically, the most desired property increases over the HK-type are in corrosion resistance, carburization resistance, creep and rupture strength and hot ductility at moderate materials cost. Alloys which have been developed to provide those improved properties fall into three general categories: improvements in the HK-base; improvements in the HP-base; and alloys of even higher total strategic element contents for the severest service.
The first significant improvement in reasonable cost alloys for high temperature applications combined with moderate increase in hot strength over standard HP grade was disclosed in U.S. Pat. No. 2,540,107 to English et al, which describes alloys of 40%-60% Ni, 22%-34% Cr, 4%-6.5% W, and
0.35%-0.75%C. An alloy of nominal composition of 48% Ni, 28% Cr and 5% W is commercially known as NA22H and was intended for service up to 2200.degree. F. as compared to the H-type alloys which corrode severely at temperatures of 2100.degree. F. or less.
Avery, U.S. Pat. No. 3,127,265, discloses the most significantly improved alloys to the present. An alloy falling within the Avery teachings contains nominally 35% Ni, 26% Cr, 15% Co, 5%W and 15% Fe, and is marketed under the trade name Supertherm. The primary improvements Provided by this alloy are increased life expectancy and lowered creep rate.
Later, in U.S. Pat. No. 3,607,250, English et al disclosed an alloy which was essentially the NA22H alloy plus about 3% Co, which is known as Super NA22H. However, while the hot corrosion resistance of the newer alloy is equal to that of Supertherm, its hot strength is inferior to Supertherm over the entire useful temperature range.
British patent No. 1,046,603, of 1965, discloses
nickel-base alloys containing 26%-38% Cr and 10%-25% W. An alloy containing nominally 48.7% Ni, 34% Cr, 16% W and impurities. This alloy has been known commercially as MoRe2 and is intended for service up to 2500.degree. F. However, MoRe2 serves to demonstrate the problem of lowered structural stability in such alloys. Carbon, nitrogen, nickel and cobalt tend to promote the stable desired matrix structure, while chromium, tungsten, molybdenum, colubium, tantalum, aluminum, titanium and other hardening and strengthening elements tend to destabilize or alter the matrix crystal structure to some extent. Therefore, this second group of elements must be somewhat limited in relation to the contents of the elements of the first group. Otherwise, hot strength, ductility, corrosion resistance or other properties will suffer. In the case of MoRe2, hot strength and corrosion resistance above about 2200.degree. F. are superior to those of Supertherm and others, but MoRe2 is exceedingly expensive and of inferior hot strength below about 2200.degree. F. It has therefore never been extensively used for commercial applications.
Also, in the mid-1950's, workers at the U.S. Naval Boiler and Turbine Laboratory began development of extremely corrosion resistant alloys of nominally 50% Ni-50% Cr and 40% Ni-60% Cr content, which were intended to resist the very corrosive effects of fuel-oils containing high amounts of sodium and vanadium compounds. These alloys had poor hot strengths, though the hot strength of the 50% Ni-50% Cr grade was slightly improved in subsequent work by the addition of about 1.5% Cb.
These very high chromium alloys and the MoRe2 alloy are examples of how hot strength may be sacrificed to obtain increased hot corrosion resistance. On the other hand, the gas turbine blade alloys are examples of sacrificing hot corrosion resistance to obtain the ultimate in hot strength at very high materials cost. However, there has remained a great need for improved hot strength in alloys approximately equal in hot corrosion resistance to those of U.S. Pat. No. 3,127,265 at moderate cost.